1. Field of the Invention
The present invention relates to a MEMS device and a portable communication terminal that includes the MEMS device.
2. Related Art
MEMS (Micro-Electro-Mechanism System) utilizing a semiconductor process is expected to be applied in various fields. For example, in the field of high-frequency circuits, applications of MEMS devices as RF switches and variable capacitors are strongly expected.
MEMS switches for high-frequency waves are roughly divided into the group of DC contact MEMS switches that can be used with DC and high-frequency waves and each have two contact points in ohmic contact with each other, and the group of capacitive switches that can be used only at high frequencies of 10 GHz or higher and each have two contact points in contact with each other via a thin dielectric film. Since consumer portable wireless devices normally use 500 MHz to 5 GHz bands, DC contact MEMS switches serve more uses.
As the related driving mechanisms for DC contact MEMS switches, electrostatic driving mechanisms have been mostly used. This is because electrostatic driving mechanisms have simple materials and structures, and processing with an electrostatic driving mechanism is easy. A typical structure of an electrostatic driving mechanism has a fixed electrode formed on a substrate and covered with a dielectric film. A contact electrode for ohmic contact is also formed on the substrate, and a conductive movable beam bridging the upper portions of the fixed electrode and the contact electrode with a weak spring is provided. A voltage is applied between the fixed electrode and the movable beam, so as to generate an electrostatic force. The contact electrode and the movable electrode of the movable beam are attracted to each other by the electrostatic force, and are brought into ohmic contact with each other. In this manner, the switch is opened and closed.
FIG. 14 schematically shows the spring force of the movable beam, the electrostatic force, and the sum of the two forces. To maintain isolation of a MEMS switch, it is necessary to move the movable beam by 2 μm to 3 μm. However, the electrostatic driving force rapidly decreases in inverse proportion to the square of the distance between the contact electrode and the movable electrode. Therefore, the spring constant becomes relatively smaller, and a high voltage of 20 V or more is constantly required as the electrostatic driving voltage. When the contact electrode and the movable electrode of the movable beam are brought into contact with each other, a strong contact pressure force is generated. However, the separating process depends only on the spring force, resulting in the problems that the separation force is very weak, the contact points are fixed, and the reliability is low. To sum up, an electrostatically-driven MEMS switch has the advantage that the contact pressure force is large, but has the disadvantages that the driving voltage is high and the separation force is weak.
Meanwhile, piezoelectric driving systems have been suggested as MEMS driving mechanisms. A piezoelectric driving system has a piezoelectric film interposed between electrode films in a movable beam as a piezoelectric driving mechanism. A movable electrode is provided on the movable beam, and a fixed electrode is provided on the substrate. The spring force of the movable beam is the linear function of the distance between the movable electrode and the fixed electrode. FIG. 15 schematically shows the spring force of the movable beam, the piezoelectric driving force, and the sum of the two forces. The piezoelectric driving force is constant in direct proportion to the voltage, and a relatively low voltage can cause a great change of the piezoelectric driving force. However, the driving force is weak, and the contact pressure force and the separation force are accordingly small. To sum up, a piezoelectrically-driven MEMS switch has the advantage that the driving voltage is low, but has the disadvantages that the contact pressure force and the separation force are weak.
To counter this problem, a hybrid driving mechanism using both electrostatic and piezoelectric forces has been suggested, with the advantages of electrostatic driving and piezoelectric driving being combined (see JP-A 8-506690 (KOKAI)). FIG. 4 of JP-A 8-506690 (KOKAI) shows the spring force of the driving beam, the electrostatic driving force, the piezoelectric driving force, and the sum of the three forces. As shown in FIG. 4 of JP-A 8-506690 (KOKAI), when the movable beam is away from the fixed electrode, driving is performed mainly by the piezoelectric force. When the movable beam is close to the fixed electrode, driving is performed mainly by the electrostatic force. Accordingly, it is possible to employ a driving beam having a greater spring constant than the spring constant used in a case where only one of the two forces is used. However, there still remains the problem of the weak separation force. To sum up, a hybrid-driven MEMS switch utilizing both electrostatic and piezoelectric forces has the advantages that the driving voltage is low and the contact pressure force is large, but has the disadvantage that the separation force is rather weak.